The cell-free network has been recently proposed to address the inter-cell interference of the conventional cellular network. In order to further improve the network capacity with low power consumption, the energy-efficient RISs can be investigated in the cell-free network. However, with the increase of the number of RIS, the number of channels to be estimated increases accordingly in the RIS assisted cell-free network. One possible solution is to exploit the multi-user correlation mentioned in [1]} to reduce the number of channels to be estimated.
